Put bluntly, Dynex is often crappy or fake media, which can be made by any one of dozens of manufacturers (unlike with say, Verbatim, where you know what you're getting every time).

It's possible that the Dynex discs you used successfully before, were made by a different manufacturer to the ones you're having trouble with now.

There's a reason I often recommend stuff like Verbatim - it's because most of the time, they're the safest option for trouble-free burning.